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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="concepts.objectwrapper"></a><a class="link" href="objectwrapper.html" title="ObjectWrapper">ObjectWrapper</a>
</h2></div></div></div>
<div class="toc"><dl class="toc">
<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.introduction">Introduction</a></span></dt>
<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen">ObjectWrapper
        Concept Requirements</a></span></dt>
<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements">TypeWrapper
        Concept Requirements</a></span></dt>
<dt><span class="section"><a href="objectwrapper.html#concepts.objectwrapper.caveat">Caveat</a></span></dt>
</dl></div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.objectwrapper.introduction"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.introduction" title="Introduction">Introduction</a>
</h3></div></div></div>
<p>
          This page defines two concepts used to describe classes which manage a
          Python objects, and which are intended to support usage with a Python-like
          syntax.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.objectwrapper.objectwrapper_concept_requiremen"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper
        Concept Requirements</a>
</h3></div></div></div>
<p>
          Models of the ObjectWrapper concept have <a class="link" href="../object_wrappers/boost_python_object_hpp.html#object_wrappers.boost_python_object_hpp.class_object" title="Class object">object</a>
          as a publicly-accessible base class, and are used to supply special construction
          behavior and/or additional convenient functionality through (often templated)
          member functions. Except when the return type R is itself an <a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper</a>,
          a member function invocation of the form
        </p>
<pre class="programlisting"><span class="identifier">x</span><span class="special">.</span><span class="identifier">some_function</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre>
<p>
          always has semantics equivalent to:
        </p>
<pre class="programlisting"><span class="identifier">extract</span><span class="special">&lt;</span><span class="identifier">R</span><span class="special">&gt;(</span><span class="identifier">x</span><span class="special">.</span><span class="identifier">attr</span><span class="special">(</span><span class="string">"some_function"</span><span class="special">)(</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)))()</span></pre>
<p>
          (see <a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">caveat</a> below).
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.objectwrapper.typewrapper_concept_requirements"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.typewrapper_concept_requirements" title="TypeWrapper Concept Requirements">TypeWrapper
        Concept Requirements</a>
</h3></div></div></div>
<p>
          TypeWrapper is a refinement of <a class="link" href="objectwrapper.html#concepts.objectwrapper.objectwrapper_concept_requiremen" title="ObjectWrapper Concept Requirements">ObjectWrapper</a>
          which is associated with a particular Python type <code class="computeroutput"><span class="identifier">X</span></code>.
          For a given TypeWrapper <code class="computeroutput"><span class="identifier">T</span></code>,
          a valid constructor expression
        </p>
<pre class="programlisting"><span class="identifier">T</span><span class="special">(</span><span class="identifier">a1</span><span class="special">,</span> <span class="identifier">a2</span><span class="special">,...</span><span class="identifier">an</span><span class="special">)</span></pre>
<p>
          builds a new T object managing the result of invoking X with arguments
          corresponding to
        </p>
<pre class="programlisting"><span class="identifier">object</span><span class="special">(</span><span class="identifier">a1</span><span class="special">),</span> <span class="identifier">object</span><span class="special">(</span><span class="identifier">a2</span><span class="special">),...</span><span class="identifier">object</span><span class="special">(</span><span class="identifier">an</span><span class="special">)</span></pre>
<p>
          . When used as arguments to wrapped C++ functions, or as the template parameter
          to <a class="link" href="../to_from_python_type_conversion.html#to_from_python_type_conversion.boost_python_extract_hpp.class_template_extract" title="Class template extract">extract&lt;&gt;</a>,
          only instances of the associated Python type will be considered a match.
        </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="concepts.objectwrapper.caveat"></a><a class="link" href="objectwrapper.html#concepts.objectwrapper.caveat" title="Caveat">Caveat</a>
</h3></div></div></div>
<p>
          The upshot of the special member function invocation rules when the return
          type is a TypeWrapper is that it is possible for the returned object to
          manage a Python object of an inappropriate type. This is not usually a
          serious problem; the worst-case result is that errors will be detected
          at runtime a little later than they might otherwise be. For an example
          of how this can occur, note that the <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">dict</a>
          member function <code class="computeroutput"><span class="identifier">items</span></code> returns
          an object of type <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">list</a>.
          Now suppose the user defines this <code class="computeroutput"><span class="identifier">dict</span></code>
          subclass in Python:
        </p>
<pre class="programlisting"><span class="special">&gt;&gt;&gt;</span> <span class="keyword">class</span> <span class="identifier">mydict</span><span class="special">(</span><span class="identifier">dict</span><span class="special">):</span>
<span class="special">...</span>     <span class="identifier">def</span> <span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">):</span>
<span class="special">...</span>         <span class="keyword">return</span> <span class="identifier">tuple</span><span class="special">(</span><span class="identifier">dict</span><span class="special">.</span><span class="identifier">items</span><span class="special">(</span><span class="identifier">self</span><span class="special">))</span> <span class="special">#</span> <span class="keyword">return</span> <span class="identifier">a</span> <span class="identifier">tuple</span>
</pre>
<p>
          Since an instance of <code class="computeroutput"><span class="identifier">mydict</span></code>
          is also an instance of <code class="computeroutput"><span class="identifier">dict</span></code>,
          when used as an argument to a wrapped C++ function, <a class="link" href="../object_wrappers.html#object_wrappers.boost_python_dict_hpp.class_dict" title="Class dict">boost::python::dict</a>
          can accept objects of Python type <code class="computeroutput"><span class="identifier">mydict</span></code>.
          Invoking <code class="computeroutput"><span class="identifier">items</span><span class="special">()</span></code>
          on this object can result in an instance of <a class="link" href="../object_wrappers/boost_python_list_hpp.html#object_wrappers.boost_python_list_hpp.class_list" title="Class list">boost::python::list</a>
          which actually holds a Python <code class="computeroutput"><span class="identifier">tuple</span></code>.
          Subsequent attempts to use <code class="computeroutput"><span class="identifier">list</span></code>
          methods (e.g. <code class="computeroutput"><span class="identifier">append</span></code>, or
          any other mutating operation) on this object will raise the same exception
          that would occur if you tried to do it from Python.
        </p>
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